1. FIELD OF THE INVENTION
The present invention relates to a screw compressor for use as an air compressor or a refrigerant compressor of an air conditioner or refrigerator. More particularly, the invention is concerned with an oil flooded screw compressor.
2. DESCRIPTION OF THE PRIOR ART
In general, a screw compressor is constituted by parts such as a casing having two intersecting bores with parallel axes, and a male rotor and a female rotor disposed in these bores so as to mesh with each other. In operation, as these rotors are driven by a suitable driving means, air or a refrigerant gas is sucked from a suction port formed in the casing into the working space defined by these rotors and the casing. The air or gas is then compressed in the working space to a higher pressure and is discharged through a discharge port formed in the casing.
The construction and operation of the screw compressor of the kind described are well known and are shown, for example, in the specification of the U.S. Pat. No. 3,423,017.
This oil flooded screw compressor, however, involves some disadvantages as will be explained hereinbelow with reference to the U.S. Pat. No. 3,423,017.
When the male and female rotors take positions as shown in FIG. 5b attached to the U.S. Pat. No. 3,423,017, both rotors make mutual contact theoretically at three points 74, 66 and 82. Usually, slight clearances are formed between the rotors in order to ensure a smooth rotation of these rotors while avoiding leak of the fluid through these clearances. In the meshing regions of these rotors, spaces 78 and 76 having ends defined by the contact points 74, 66 and 66, 82 are formed respectively. One 78 of these spaces has a channel form with its volume increasing with the rotation of the rotor. This space 78 is communicated at the other end of the rotor with a suction chamber (not shown), as will be seen from FIGS. 5b, 5c and 5d of the U.S. Pat. No. 3,423,017.
The other space 76 has a channel form with a decreasing volume, and is surrounded by the surfaces of lobes of both rotors. This space is shielded from the outside except for the end portions of the rotor. In the state shown in FIG. 5b, the space 78 is communicated with the suction chamber. Therefore, any leak of the compressed gas of high pressure into this space 78 leads to a loss of the compressed gas. Therefore, the discharge port 36 shown by one-dot-and-dash line in FIG. 5b has to be positioned so as not to communicate with the space 78.
On the other hand, in order to decrease the discharge loss, there is a demand for increasing the area of the discharge port 36 as much as possible.
In order to compromise these two requirements which are generally incompatible, a baffle means is provided at the center of the discharge port 36. Usually, this baffle means has a contour which conforms with the locus of the contact point 74 and the locus of the contact point 66.
According to this arrangement, in the period of rotation of the rotors from the position shown in FIG. 5b to the position shown in FIG. 5c, the space 76 is isolated from the discharge port 36 and is closed completely. Since the volume of this closed space is decreased with the rotation of the rotors, the fluid confined in this closed space is over compressed to an abnormally high pressure. This causes not only noise and vibration of the rotor but burdens the rotors excessively, often resulting in a breakdown of the rotor and/or shortening of the life of the bearings.
In the described prior art shown in the U.S. Pat. No. 3,423,017, in order to avoid the confinement of the fluid into the closed space 76, recesses 106 and 108 are provided at the crests of the lobes as shown in FIGS. 6 and 7a to 7d. These recesses 106 and 108 are somewhat effective in eliminating the confinement of the fluid. It is, however, not allowed to increase the sizes of these recesses unlimitedly for the following reason. Namely, if the recess is formed at the crests of the lobe, the fluid leaks through this recess even in other than the period in which the confinement takes place. This obviously causes a serious loss of compressed fluid. Thus, the size of the recesses 106, 108 is naturally limited. The provision of the recess at the crest of the lobe is not preferred also from the view point of productivity, because the life of the manufacturing tool is shortened due to the complicated configuration of the lobe crest.
Japanese Patent Laid-Open No. 58-214693 also proposes a solution to the problem incurred by the closed space 76. This solution, however, is still unsatisfactory in that the pressure rise in the space 76 cannot be prevented perfectly because the closed space 76 is allowed to be communication with the low-pressure side only gradually with the rotation of the rotors.